There are already known sewing machines comprising a needle bar and means for imparting reciprocating longitudinal movements thereto, means for securing a needle on this bar and supplying thread to this needle, a loop holder for taking hold of the thread introduced by the needle and causing it to pass around a shuttle spool whose thread thus passes through this loop so as to form a seam stitch, a mechanical transporter for displacing the fabric to be sewn from one completed seam stitch to the following point to be sewn, means being provided for slackening the thread issuing from the spool, at the time of each seam stitch formation.